Synthesis of COF-SO3H immobilized on manganese ferrite nanoparticles as an efficient nanocomposite in the preparation of spirooxindoles

The synthesis of sulfonamide-functionalized magnetic porous nanocomposites is highly significant in chemistry due to their exceptional properties and potential as catalysts. COFs are a new class of organic porous polymers and have significant advantages such as low density, high chemical and thermal stability, and mechanical strength. Therefore, we decided to synthesize COFs based on magnetic nanoparticles, by doing so, we can also prevent the agglomeration of MnFe2O4. MnFe2O4@COF–SO3H possesses a large specific surface area, supermagnetism, and is acidic, making it an optimal catalyst for organic reactions. This particular catalyst was effectively employed in the green and rapid synthesis of various spiro-pyrano chromenes, while several analytical techniques were utilized to analyze its structural integrity and functional groups. The role of a specific site of MnFe2O4@COF–SO3H was confirmed through different control experiments in a one-pot reaction mechanism. It was determined that MnFe2O4@COF–SO3H acts as a bifunctional acid–base catalyst in the one-pot preparation of spirooxindole derivatives. The formation of a spiro skeleton in the multicomponent reaction involved the construction of three new σ bonds (one C–O bond and two C–C bonds) within a single process. The efficiency of the MnFe2O4@COF–SO3H complex is investigated in the synthesis of spirooxindoles of malononitrile, and various isatins with 1,3‐dicarbonyles. The nanocatalyst demonstrated excellent catalytic activity that gave the corresponding coupling products good to excellent yields. Furthermore, the heterogeneous magnetic nanocatalyst used in this study demonstrated recoverability after five cycles with minimal loss of activity.


Experimental section
Structural analysis of the MnFe 2 O 4 @COF-SO 3 H nanocatalyst Initially, the synthesis of manganese ferrites (MnFe 2 O 4 ) nanoparticles involved the combination of Fe(III) salt and Mn(II) salt in an alkaline solution, resulting in the precipitation of spinel ferrite, MnFe 2 O 4 , from the solution.
Figure 2a-c illustrate the FT-IR spectra of MnFe 2 O 4 , MnFe 2 O 4 @COF, and MnFe 2 O 4 @COF-SO 3 H.The FT-IR spectrum depicted in Fig. 2a displays two distinct vibration bands at approximately 578 and 480 cm −1 , which correspond to the Fe-O and Mn-O vibrational modes in manganese ferrite, respectively.This confirms the formation of a single-phase MnFe 2 O 4 as no other metal oxide bands are observed within the 400-1000 cm −1 range.The broad peak observed at 3426 cm −1 is attributed to the stretching vibration of N-H functional groups or hydrogen-bonded surface water molecules.
The FT-IR spectra of MnFe 2 O 4 @COF revealed distinct differences when compared to those of MnFe 2 O 4 nanoparticles and monomers (Fig. 2b).The characteristic aromatic rings can be detected at 1548 and 1474 cm −1 , indicating the presence of C=C and C=N vibrations, while the absence of the C=O vibration of TPA at 1694 cm −1  confirmed that the COF shells were successfully formed through a Schiff-base reaction and coated onto the surface of MnFe 2 O 4 nanoparticles.The spectrum of MnFe 2 O 4 @COF-SO 3 H shows bands in the range of 1020-1150 cm −1 , which correspond to the O=S=O asymmetric and symmetric stretching modes (Fig. 2c).The peak intensity in the range of 3400 has considerably increased, providing strong evidence for the presence of the acidic group.These findings indicate that -SO 3 H groups have been effectively incorporated into the primary polymeric framework of COF.Furthermore, it is worth noting that the successful functionalization of MnFe 2 O 4 @COF with -SO 3 H groups can be confirmed through EDX analysis.
Figure 3 presents the XRD spectra of MnFe 2 O 4 and MnFe 2 O 4 @COF-SO 3 H.Subfigure (a) of Fig. 3 displays the XRD pattern of MnFe 2 O 4 nanoparticles, showing well-defined and intense peaks indicating good crystallinity.The diffraction peaks observed at 18.68°, 29.69°, 34.97°, 42.53°, 56.18° and 61.61° correspond to the lattice planes of (111), ( 220), (311), (400), ( 511) and (440), respectively, in agreement with standard JCPDS (No. 01-074-2403).These peaks correspond to specific lattice planes, confirming the phase purity of MnFe 2 O 4 .No impurity peaks were detected.The grain size for the high-intensity peaks was approximately 20 nm, determined using Scherrer's equation.In the XRD pattern (Fig. 3b), broad peaks between 2θ values of 15°-30° suggest the amorphous nature of the porous organic polymer.However, the surface structure of the material remained relatively unchanged before and after modification with sulfonic acid groups, indicating that the original structure was not significantly altered.
The MnFe 2 O 4 and MnFe 2 O 4 @COF-SO 3 H materials were examined using FE-SEM analysis to investigate their morphological and structural properties (Fig. 4a, b).The obtained images indicate that the particles exhibit a nearly spherical morphology.It is evident from the images that both MnFe 2 O 4 and MnFe 2 O 4 @COF-SO 3 H possess nano-sized structures, with average sizes of approximately 28 nm and 35 nm, respectively.Additionally, partial aggregation can be observed, which is beneficial for applications requiring high electron transfer conductivity.
To further examine the catalyst's morphology, a TEM analysis was conducted.The TEM images of the catalyst under investigation indicate a nearly uniform distribution of MnFe 2 O 4 nanoparticles within the acidic COF, as shown in Fig. 4c.
The EDX results confirm the successful preparation of pure MnFe 2 O 4 nanoparticles without any significant impurities detected in the sample (Fig. 5a).Furthermore, the presence of Mn, N, Fe, O, C, and S species in the MnFe 2 O 4 @COF-SO 3 H material is confirmed by EDX analysis (Fig. 5b).The sulfur loading content is determined to be 18 wt%, providing further evidence that sulfonic acid was successfully loaded onto the polymer surface.EDS mapping images demonstrate that all elements are uniformly dispersed within the polymer network in both samples (Fig. 6a, b).
Porous structures have a critical role in regulating catalytic properties within an academic context.Additionally, these structures often display intricate and interconnected three-dimensional geometries.to determine the porous characteristics of the sample, N 2 adsorption-desorption analysis was conducted at 77 K (Fig. 7).The N 2 isotherm of the MnFe 2 O 4 @COF-SO 3 H catalyst confirmed a mesoporous structure, indicated by its type IV isotherm.Also, H2 type hysteresis loop in the relative pressure ranges from 0.3 to 1.00, is attributed to mesopore materials.
The specific surface area of the catalyst was determined to be approximately 12.671 m 2 g −1 .Furthermore, the composite exhibited a pore size distribution of approximately 8.60 nm and a total pore volume of 0.027 cm 3 g −1 .Therefore, the fabricated MnFe 2 O 4 @COF-SO 3 H catalyst demonstrated a suitable pore structure and a favorable surface area, which could significantly enhance its catalytic efficiency.
The magnetic hysteresis curves of MnFe 2 O 4 and MnFe 2 O 4 @COF at room temperature are illustrated in Fig. 8.The saturation magnetization of these materials was measured as 17.58 and 15.90 emu g −1 , respectively.The lower saturation magnetization observed can be attributed to various factors, including crystalline nature, particle size, particle arrangement, adsorbed layer of molecules on the particle surface, and random canting of  www.nature.com/scientificreports/particle surface spins 36 .This dependence is also influenced by the concentration of trivalent and divalent cations in the tetrahedral and octahedral sites 37 .Furthermore, nanosized particles with a high surface-to-volume ratio exhibit decreased saturation magnetization.The thermal behavior of MnFe 2 O 4 @COF-SO 3 H nanoparticles was analyzed with TGA.The MnFe 2 O 4 nanoparticles showed a weight loss of 8.1% up to 500 °C, which was attributed to the evaporation and breakdown of small organic compounds, as seen in Fig. 9.In contrast, MnFe 2 O 4 @COF displayed two thermal degradation stages   on its hydrogenation curve.The initial weight loss of 1% occurred within the temperature range of 100-250 °C, resulting from the evaporation of bound water and volatile small organic compounds.Subsequently, a gradual weight loss took place between 250 and 500 °C as a consequence of the sulfonic acid group being broken down first, followed by the decomposition of COF's organic structure.
The efficacy of synthesized nanocatalysts in organic reactions was demonstrated through the utilization of COF-SO 3 H immobilized on MnFe 2 O 4 nanoparticles [MnFe 2 O 4 @COF-SO 3 H] as an effective and reusable nanocatalyst for the production of spirooxindoles.This was achieved by coupling 1,3-dicarbonyls with malononitrile and various isatins.To evaluate the reaction, dimedone, malononitrile, and isatins were utilized as model substrates in a variety of solvents (Fig. 10).The results revealed that the efficacy of the reaction was impacted by different solvents.Low yields (53-58%) were obtained when acetonitrile and dichloromethane were used as solvents, whereas water, DMF, and DMSO improved yields.Ethanol was found to be the most effective solvent, producing a yield of 98%, exceeding all other solvents tested.Without solvent, the yield decreased to 39% for model reactions.
The use of isatin in electrophilic reactions with diverse nucleophiles is considered a fundamental approach for generating multicomponent reactions.In addition to its role as a solvent, protic solvents can facilitate the enolization of dimedone by forming hydrogen bonds with the OH group.This, in turn, enhances the nucleophilic properties of the methylene carbon (C-2) of dimedone and results in an accelerated reaction rate 38,39 .
Several experiments were carried out to regulate the quantity of catalyst employed.These experiments revealed that augmenting the catalyst quantity from 5 to 15 mol% resulted in a yield increase from 80 to 98%.However, employing a larger quantity of nanocatalysts (25 mol%) did not enhance the reaction yield, as evidenced in Fig. 11.

Proposed mechanism
In Scheme 2, we have a way to make spiro compounds and it involves a process called the catalytic cycle.To create spirooxindole, we use isatin, cyclic 1,3-diketone, and malononitrile in a three-component reaction.The process happens in two steps: First, the SO 3 H nanocatalyst binds to the oxygen atom of the carbonyl group through electrostatic attraction.At the same time, lone pairs of the amino group of COF remove the acidic hydrogen from malononitrile.Then, carbanion attacks the carbonyl group of isatin and produces isatylidenemalononitrile (I) through a Knoevenagel condensation.In the following step, the carbanion reacts with the activated double bond of cyclic 1,3-dicarbonyl (II) via Michael addition and produces intermediate (III).Afterward, an intramolecular nucleophilic addition reaction forms an intermediate (IV).Lastly, we get our final product by an isomerization process.

Reusability of the catalyst
To assess the possibility of reusing the catalyst, it was collected after the reaction process using an external permanent magnet.Subsequently, the catalyst was subjected to multiple washes with ethanol and dried at a temperature of 50 °C before its use in subsequent cycles.The findings indicate that the catalyst was effectively utilized in five consecutive reaction cycles with no notable decline in product yield, with initial yields of 98% and 85% by the fifth cycle (Fig. 12).
This catalyst was prepared using simple salts and cheap materials.Also, according to the economic approach, it can be used up to 5 times in the reaction with the greatest effect.
Results from this study and other studies on the model reaction show that our method, which uses a MnFe 2 O 4 @COF-SO 3 H catalyst, produces a higher yield in less time (Table 3).As depicted in Table 3, the use of other catalysts requires a longer time, lower efficiency, and higher costs [48][49][50] .In contrast, utilizing MnFe 2 O 4 @ COF-SO 3 H solves these problems considering that it is possible to collect the catalyst with an external magnet.The recovery of the catalyst is easily possible without loss of efficiency, resulting in significant cost reductions.Moreover, this approach is also considered environmentally favorable.www.nature.com/scientificreports/

Substances and methods
All reagents and solvents used in the study were commercially purchased and did not undergo any additional purification.Fourier transform infrared (FT-IR) spectroscopy was conducted using a Nicolet Magna-400 spectrometer with KBr pellets.1H NMR data were collected in DMSO-d6 using a Bruker DRX-400 spectrometer and tetramethylsilane as the internal reference.XRD patterns were recorded using a Philips diffractometer with monochromatized Cu K radiation.The morphology of the nanoparticles was analyzed using field emission www.nature.com/scientificreports/scanning electron microscopy (FE-SEM) with model MIRA3.An Arya Electron Optic instrument was utilized in the academic analysis of the catalyst through the use of electron dispersive X-rays (EDX) and mapping techniques.The surface area measurement was conducted using the Brunauer Emmett Teller (BET) method, which involved nitrogen adsorption analyzed by a mechanized gas adsorption analyzer (Belsorp mini II, Microtrac Bel Corp).The microscopic morphology of the nanoparticles was observed using a Philips transmission electron microscope (TEM) operating at 100 Kv.At Iran's Kashan University, the magnetic characteristics of materials were evaluated with a magnetometer (VSM, PPMS-9T) at a temperature of 300 K. Thermogravimetric analysis (TGA) was performed on a Mettler TA4000 system TG-50, utilizing a heating rate of 10 K min1 in an N2 atmosphere.The Yanagimoto micro melting point device was employed to measure the melting points without any correction.
To monitor the reaction and determine substrate purity, thin-layer chromatography (TLC) was carried out on silica-gel polygram SILG/UV 254 plates provided by the Merck Company.

Synthesis of catalyst
Preparation of modified MnFe 2 O 4 nanoparticles After 30 min of nitrogen gas bubbling in 200 mL of purified, deoxygenated water, 5 g of Mn(NO 3 ) 2 •4H 2 O and 14 g of Fe(NO 3 ) 3 •6H 2 O were dissolved in ultrapure water with vigorous mechanical stirring.The aforementioned mixture was then stirred while 2.0 M NaOH solution was added dropwise until the pH reached 11.After that, the mixture was heated to 100 °C and maintained there for 2 h.In an external magnetic field, a black precipitate was gathered and then cleaned with ultrapure water.To get rid of the contaminants connected with the operations (such as OH − , NO 3− , and Na + ), this washing was done three times.After freeze-drying, pure MnFe 2 O 4 nanoparticles were finally produced.

Preparation of MnFe 2 O 4 @COF
The synthesis of magnetic covalent organic frameworks (MnFe 2 O 4 @COF) involved dissolving 0.20 g of MnFe 2 O 4 nanoparticles in a solution of 50 mL DMSO, 2 mmol of melamine (MA), and 3 mmol of terephthalaldehyde (TPA).Following a 30-min sonication of the combination, the homogeneous black suspension was placed in a stainless-steel autoclave lined with Teflon, which was then heated to 180 °C for a reaction time of 24 h.The produced MnFe 2 O 4 @COF was magnet-separated, followed by three rounds of washing in tetrahydrofuran, anhydrous methanol, and dichloromethane.The next step was to vacuum-dry the produced MnFe 2 O 4 @COF at 50 °C.

Synthesis of MnFe 2 O 4 @COF-SO 3 H
Chlorosulfonic acid was utilized to sulfonate the MnFe 2 O 4 @COF.A standard synthesis involved suspending 0.5 g of the MnFe 2 O 4 @COF in 20 ml chloroform in a 25-mL round-bottomed flask, followed by the dropwise addition of 2 ml of chlorosulfonic in CH 2 Cl 2 (10 mL) throughout 2 h at room temperature.The catalyst obtained was subjected to multiple washes with chloroform and subsequently dried for 24 h at 60 °C in an oven.

Analysis and characterization of the synthesized compounds
Compound 4f exhibits an absorption signal at 3439 cm −1 in its IR spectrum, indicating the presence of an NH group in the molecule's structure.The absorption peaks observed at 1782, 1726, and 1665 cm −1 can be assigned to the stretching vibrations of carbonyl groups.Additionally, a C=C stretching band is observed at 1622 cm −1 .When analyzing the compound's 1HNMR spectrum, a singlet signal is detected at δ = 10.93 ppm for the NH proton.The hydrogens of aromatic moieties produce signals within the range of δ = 7.53-6.89ppm.In dimedone, the hydrogens of 2CH 2 appear as two doublet peaks at δ = 2.20 and δ = 2.06 ppm with 16 Hz, along with a single peak at δ = 2.95 ppm.The two sharp singlet peaks observed at δ = 1.13 and δ = 1.09 ppm can be attributed to the presence of the 2CH 3 groups in the dimedone moiety.

Conclusion
This report discussed the preparation of COF and their composites with magnetic nanoparticles (MnFe 2 O 4 ).These materials possess distinct characteristics that make them as viable options for material science applications.Firstly, the synthesis process follows a one-pot approach.Secondly, the materials offer customizable porosity.Thirdly, the starting components of the materials are inexpensive.Fourthly, the catalyst can be isolated with an external magnet.Lastly, nanocomposites with elevated amounts of nitrogen have been successfully produced.MnFe 2 O 4 @COF-SO 3 H serves as a reusable and efficient nanocatalyst for the synthesis of spirooxindoles, comparable to other commonly used catalysts.The proposed method suggests various advantages, such as simplicity, high yields, shorter reaction time, reduced environmental impact, and a safe and cost-effective starting procedure (no toxic solvents were used in the reaction or work-up procedures).Consequently, this method is valuable and appealing for the preparation of these important compounds.Additionally, considering the abundance of isatins and 1,3-dicarbonyl compounds, this approach holds the potential for generating libraries with significant diversity.Therefore, it is anticipated that this method will find widespread application in drug discovery and combinatorial chemistry.

Figure 10 .
Figure 10.The impact of various solvents on the preparation of oxindole using MnFe 2 O 4 @COF-SO 3 H.

Table 1 .
The optimization of reaction conditions aimed at generating spiro-pyrano chromen.a Isolated yields.

Table 3 .
Comparison the various catalysts for the synthesis of spirooxindole compounds.